1. Field of the Invention
This invention relates to a retro-focus (reverse telephoto) type lens, and particularly to a retro-focus type lens suitable, for example, as a projection lens for a color liquid crystal projection TV set designed such that a plurality of images having different kinds of color information are synthesized by a synthesizing mirror and thereafter enlargedly projected onto a screen surface.
2. Related Background Art
There have heretofore been proposed various color liquid crystal projection type TV sets designed such that images being displayed on a plurality of color liquid crystals (liquid crystal light bulbs) are optically superposed one upon another and projected onto a screen surface by a projection lens.
FIG. 9 of the accompanying drawings shows an assembly of the essential portions of a color liquid crystal projection TV set in which images formed on popular color liquid crystals are projected onto a screen surface S.
In FIG. 9, the reference numeral 1 designates a white light source emitting a collimated light beam. The reference characters 2a, 2b and 2c denote liquid crystal display elements for red, green and blue, respectively. Projected images are displayed on these liquid crystal display elements. The reference characters 3a and 3b designate ordinary mirrors, and the reference numeral 4 denotes a red reflecting dichroic mirror. Red light illuminates the liquid crystal display element 2a for red. The reference numeral 5 designates a green reflecting dichroic mirror. Green light illuminates the liquid crystal display element 2b for green.
The liquid crystal display element 2c for blue is illuminated by blue light passed through the red reflecting dichroic mirror 4 and the green reflecting dichroic mirror 5. The reference numeral 6 denotes a blue transmitting dichroic mirror. The reference numeral 7 designates a projection lens.
In FIG. 9, the white light from the white light source 1 is color-resolved into red, green and blue color lights by the dichroic mirrors 4 and 5, the liquid crystal display elements 2a, 2b and 2c for red, green and blue are illuminated by these red, green and blue color lights, respectively, and the images of the liquid crystal display elements 2a, 2b and 2c based on these color lights are superposed on the screen surface by the projection lens 7, whereby a color image is obtained.
From the necessity of disposing various kinds of optical members such as reflecting mirrors and dichroic mirrors between the last lens surface to the liquid crystal display elements (the back focal length), a retro-focus type lens having a long back focal length is often used as the projection lens in such a construction.
Generally, a retro-focus type lens comprises a lens construction in which a lens unit of negative refractive power is disposed on the object side (the conjugate point side having a long distance) and a lens unit of positive refractive power is disposed on the image plane side (the conjugate point side having a short distance). This leads to the feature that a relatively long back focal length can be easily obtained.
However, the lens construction is asymmetrical, and this leads to the tendency that asymmetrical aberrations such as distortion and astigmatism are much created.
Where a retro-focus type lens is used in a color liquid crystal projection TV set, a color resolving dichroic mirror is disposed on the image plane side as shown in FIG. 9 and therefore, to eliminate color irregularity and effect the color reproduction of the entire picture plane well, it is necessary that the angle of incidence of the light beam onto the dichroic mirror be made substantially equal over the entire picture plane. That is, it is necessary that the lens system be brought into a state approximate to that of a so-called emergence telecentric system in which a chief ray emerging to the image plane side becomes substantially parallel to the optical axis.
However, if the lens system is made into an emergence telecentric system, the off-axial light beam will come to enter the lens unit of positive refractive power on the image plane side at a high position from the optic axis. This gives rise to the problem that barrel type (negative) distortion and flare are much created and it is very difficult to obtain a good projected image.
So, the applicant has previously proposed in Japanese Patent Application No. 1-286058 (Laid-Open No. 3-145613) a retro-focus type lens having a lens construction approximate to an emergence telecentric system suitable as a projection lens for a color liquid crystal projection TV set in which the lens construction is set appropriately, whereby the above-mentioned aberrations such as distortion and flare are corrected well.
Japanese Laid-Open Patent Application No. 54-139725 is known as a retro-focus type photographic lens provided with an aspherical surface.